On the mathematics of beauty: beautiful images

On the mathematics of beauty: beautiful images A. M. Khalili 1 Abstract The question of beauty has inspired philosophers and scientists for centuries. Today, the study of aesthetics is an active research topic in fields as diverse as computer science, neuroscience, and psychology. In this paper, we will study the simplest kind of beauty which can be found in simple visual patterns. The proposed approach shows that aesthetically appealing patterns deliver higher amount of information over multiple levels in comparison with less aesthetically appealing patterns when the same amount of energy is used. The proposed approach is used to classify aesthetically appealing patterns. Keywords Evolutionary Art, Image Aesthetic Assessment, Information Theory, Statistical Mechanics. INTRODUCTION The study of aesthetics started with the work of Plato, and today it is an active research topic in fields as diverse as neuroscience [1], psychology [2], and computer science. Baumgarten [3] suggested that aesthetic appreciation is the result of objective reasoning. Hume [4] took the opposing view that aesthetic appreciation is due to induced feelings. Kant argued that there is a universality aspect to aesthetic [5]. Shelley et al. [6] studied the influence of subjective versus objective factors in aesthetic appreciation. Recent works on empirical aesthetics [7] show that there is a general agreement on what is considered beautiful and what isn t, despite the subjectivity of aesthetic appeal. Predicting the aesthetic appeal of images is beneficial for many applications, such as recommendation and retrieval in multimedia systems. The development of a model of aesthetic judgement is also a major challenge in evolutionary art [8], [9], where only images with high aesthetic quality should be generated. The development of the social media and the fast growth in visual media content, have increased the requirement of aesthetic assessment systems. Automating the aesthetic judgements is still an open problem, and the development of models of aesthetic judgement is the main challenge. Datta et al. [10] extracted 56 visual features from an image and used them to train a statistical model to classify the images as beautiful or ugly. Some examples of the used features include: mean pixel intensity, relative colour frequencies, mean pixel hue, and mean pixel saturation. They also used photographic rules of thumb such as the rule-of-thirds. Other features related to aspect ratio, texture, and low depth-of-field were also used. Ke et al. [11] used features that describe the spatial distribution of colour, edges, brightness, and blur. Aydin et al. [12] computed perceptually calibrated ratings for a set of meaningful and fundamental aesthetic attributes such as depth, sharpness, tone, and clarity, which together form an aesthetic signature of the image. Recent works have also investigated the role of photographic composition [13], [14], [15], [16], colour compatibility [17], [18], [19], and the use of other features such as object types in the scene [20]. Recently, convolutional neural networks (CNNs), which can automatically learn the aesthetic features, have been applied to the aesthetic quality assessment problem [21], [22], [23], [24], promising results were reported. Birkhoff [25] proposed an information theory approach to aesthetic, he used a mathematic based aesthetic measure, where the measure of aesthetic quality is in a direct relation to the degree of order O, and in a reverse relation to the complexity C, M = O/C. Eysenck [26], [27], [28] conducted a series of experiments on Birkhoff s model, he argued that the aesthetic measure have to be in a direct relation to the complexity rather than an inverse relation M = O C. Javid et al. [29] conducted a survey on the use of entropy to quantify order and complexity, they also proposed a computational measure of complexity, their measure is based on the information gain from specifying the spatial 1

distribution of pixels and their uniformity and non-uniformity. Herbert Franke [30] proposed a model based on psychological experiments which showed that working memory can t take more than 16 bits/sec of visual information. He argued that artists should provide an information flow of about 16 bits/sec for their works to be perceived as aesthetically appealing and harmonious. In music, Manaris et al. [31], Investigated Arnheim s view [32], [33], and [34] that artists tend to produce art that makes a balance between chaos and monotony, they showed the results of applying the Zipf s Law to music, they proposed a large group of metrics based on the Zipf s Law to measure the distribution of various parameters in music, such as duration, pitch, consonance, melodic intervals, and harmonic. They applied these metrics to a large set of pieces, their results show that metrics based on the Zipf s Law capture essential aspects of music aesthetics. Simple Zipf metrics have a main limitation, they examine the piece as a whole, and ignore some significant details. For example, sorting a piece s notes in a different order will produce an unpleasant musical artifact that has the same distribution of the original piece. Therefore fractal metrics were used in [31] to cope with the limitation of simple metric. The fractal metric captures how many subdivisions of the piece have the same distribution at many levels of granularity. For example, the simple pitch metric was recursively applied to the piece s half subdivisions, quarter subdivisions, etc. However, as stated by the authors this law is a necessary but not sufficient law. Datasets such as [35], [36], [37], [38] and [39] are collected from community where images are uploaded and scored in response to photographic challenges. The main limitation of these datasets is that the images are very rich, diverse, and highly subjective, which will make the aesthetic assessment process very complicated. In this paper a novel approach to classify aesthetically appealing images will be presented. The main contribution of this paper is showing that aesthetically appealing patterns deliver higher amount of information over multiple levels in comparison with less aesthetically appealing patterns when the same amount of energy is used. A new dataset with very simple visual patterns will be also proposed to simplify the assessment process. The complete dataset can be found at [40]. Proposed Approach We propose a new dataset for images aesthetic assessment. The dataset contains simple visual patterns generated by the same physical process. Propagation of waves inside geometrical structures could produce very interesting interference patterns, particularly inside symmetrical shapes. The resulted pattern represents the wave interference pattern inside a closed box. Three waves were initiated at the center of the box at different time instances. The first wave was initiated when the value of the counter is 1, the second wave was initiated when the value of the counter is 5000, and the third wave was initiated when the value of the counter is 10000. The size of the images is 116x116 pixels. No aesthetic score is available for the current version of the dataset. To isolate the effect of the colours in the assessment process, a grayscale version of the image will be used in the assessment process, the coloured version of the image is only shown for illustration purposes. We will show two groups of images from the proposed dataset, the first one represents more aesthetically appealing images Fig. 1, and the second one represents less aesthetically appealing images Fig. 2. The two groups are classified by the authors. 2

Fig. 1. Images in the first group. 3

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Fig. 2. Images in the second group. In this section, the simplest kind of beauty that can be found in simple visual patterns will be studied. The transition pattern of the images will be studied by analysing the distribution of the gradient of the images. 5

It was shown in [42] that aesthetically appealing patterns have a balance between randomness and regularity, and aesthetically appealing patterns are those which are closer to this optimal point. The entropy was used as a measure of randomness and the energy was used as a measure of regularity. It was also shown that among all the patterns that have the same energy, the aesthetically appealing ones have higher entropy over multiple levels. The resulted distribution of this optimization process between randomness and regularity can be uniquely identified by maximizing the entropy giving that the energy levels are constant, the number of values are constant, and the total energy is constant. In this paper we will use the same approach to study the aesthetic appeal of visual patterns. To analyse the images of Fig.1 and Fig. 2, if we start from the centre of the image to the boundary, we notice that the number of transitions between lighter and darker values is larger for images in Fig. 1, furthermore; the intensity of the transitions is higher. This will result in increasing the high energy part of the distribution of the gradient of the image. Moreover, we notice that the high energy part of the distributions of the images of Fig.1 is larger than the high energy part of the distributions of the images in Fig. 2 when both have the same amount of energy, and since the most part of the distribution is located in the low energy region, this means that increasing the high energy part of the distribution will increase the entropy. Fig.4 shows the distribution of the gradient of one image in the dataset, the same distribution has shown up for all the images in the dataset. We can observe the similarity between the resulted distribution and the Maxwell-Boltzmann distribution which is shown in Fig. 3. Furthermore, using the above formulation, our problem now is exactly the same problem that Boltzmann [43] solved to derive the distribution of the energies of gas particles at equilibrium. Boltzmann argued that the Maxwell-Boltzmann distribution [44, 45] is the most probable distribution and it will arise by maximizing the multiplicity (which is the number of ways the particles can be arranged) giving that the number of particles is constant as described by (1), the energy levels that the particles can take are constant as described by (2), and the energy is constant as described by (3). The multiplicity is given by (4), and the entropy is given by (5). i n i = Constant (1) ε 1, ε 2,, ε n Constant (2) Energy = i n i ε i = Constant (3) Ω = N! n 1!n 2!.n n! (4) Entropy = log (Ω) (5) Where N is the total number of particles, n i is the number of particles at the ε i energy level. Maximizing the entropy is equivalent to maximizing the multiplicity. By taking ln(ω) we get ln (Ω) = ln(n!) ln (n i!) i (6) Using Stirling approximation we get ln(ω) = N ln(n) N [n i ln (n i ) n i ] i (7) The Maxwell-Boltzmann distribution gives the number of particles at each energy level. Using the Lagrange multiplier method to maximize the entropy using the constraints in (1), (2), and (3) we get n i = e α βε i (8) Where α, β are the Lagrange multipliers. The distribution in 3D and 2D spaces can be written in the form given by (9) and (10) respectively, and the distribution is shown in Fig.3. f(v) = ( m 2πkT ) 3 2 4πv 2 e mv2 2kT (9) 6

f(v) = ( m mv 2 ) 2πv e 2kT (10) 2πkT Where v is the speed of the particle, m is the mass of the particle, T is the temperature and k is Boltzmann constant. Fig. 3. The Maxwell-Boltzmann distribution for different temperature values. Similarly, in our problem, the energy levels ε 1, ε 2,, ε n are the values which the pixels can take, they will be 0, 1, 2,, 255 for grayscale images. These energy levels must be constant as described in (11), n i is the number of pixels at the energy level ε i, the total number of pixels should also be constant as described in (12). Finally, the total energy which is given by (13) must also be constant. ε 1, ε 2,, ε n Constant (11) i n i = Constant (12) Energy = i n i ε i = Constant (13) The constraints given in (11), (12), and (13) are exactly the same constraints used by Boltzmann to derive the Maxwell-Boltzmann distribution, and by maximizing the entropy, the same distribution given by (8)-(10) will arise. Maximizing the entropy will result in a flat distribution; however, the constant energy constraint will produce a balance between order and randomness. Maximizing the entropy using constant energy can then be seen as delivering the highest possible amount of information using the same amount of energy. Fig. 4 shows the distribution of the gradient of an image in the dataset. Fig. 5 shows the distribution of the gradient of the gradient of the same image. 7

Fig. 4. The distribution of the gradient of one image in the dataset. Fig. 5. The distribution of the gradient of the gradient of one image in the dataset. The same distribution has appeared for all the gradient of the images, and the gradient of the gradient of the images, which may suggest that the same law must be satisfied at each level. In [42] the multiple levels approach was also used to cope with energy and entropy limitation in representing the spatial arrangement of the pieces, where the structure of the piece was used to represent different levels; however, due to the complexity of the structure of the visual patterns, the gradient over multiple levels will be used to represent the spatial arrangement of the visual patterns, where the first level represents the image, the second level represents the gradient of the image, and the third level represents the gradient of the gradient of the image. The measures of aesthetic quality M states that the sum of the entropies of the three levels should be maximum. The measure is given by (14) M = Entropy(Li) i (14) L 1 is the image, L 2 is the gradient of the image, and L 3 is the gradient of the gradient of the image. Entropy is Shannon entropy, and the energy of the three levels must be the same. Fig.6 shows the M values of images in Fig.1 and Fig.2 with additional images in the same category. 8

Fig. 6. The M values of images in Fig.1 and Fig.2. However, comparing images that have the same energy at each level is rather limited, furthermore the above analysis doesn t say anything about the relation between the energies of different levels. Fig. 7 shows the sum of the distances between the energies of different levels for images in Fig.1 and Fig.2. Fig. 7. The sum of the distances between the energies of different levels for images in Fig.1 and Fig.2. The blue circles represent the images of Fig. 1, and the red stars represent the images of Fig. 2 along with other images in the same category. The distances of aesthetically appealing images are different from the distances of the less aesthetically appealing images. To relax the above constraint and to be able to compare images that have the same first level energy only, the aesthetically appealing images at different energy levels of Fig.1 are used as reference images, and the distances between the energies of the tested image should be as close as possible to the distances of the reference image R i as described by (15), furthermore; the equation described by (14) should be also satisfied. In other 9

words, M should be maximized and Md should be minimized. Md = Distance(Ri) i Distance(Li) i (15) Where Distance(R i) is the distance between the energy of the ith level and the energy of the i+1 level, and the energy of the first level only should be the same. The metrics will be calculated on the centre part of image since it gets most of the attention, where 20 pixels from each side of the image will be neglected. Fig.8 shows the combination of the two metrics where the sum of the entropies and the energies of the three levels is shown after scaling each energy and entropy to value between 0 and 1. Fig. 8. The sum of the entropies and energies of the three levels of images in Fig.1 and Fig.2. To further test the proposed approach, we will test it on the set proposed in [46], [47]. Fig. 9 shows the patterns of the set, the first two lines represent asymmetrical patterns; the last two lines represent symmetrical patterns. Fifty-five persons rated the patterns, the patterns start from not beautiful (left) to beautiful (right). The number next to each pattern in Fig. 10, Fig. 11, and Fig 12 represents the line number and the position of the pattern in the line (starting from left to right). For instance, 43 is the third pattern in line four. Fig. 10 shows the energy and the entropy of the first level, the results show that the symmetrical patterns of line 3 and line 4 have higher entropy than the asymmetrical patterns when the same energy is used. This matches with the rating given by the fifty-five persons and with several studies [48-51] that showed consistent preferences for symmetry. The patterns 41, 42, and 43 have roughly the same energy, but the entropy of 43 is larger than the entropy of 42, which is larger than the entropy of 41. Fig. 11 shows the sum of the entropies of the first two levels, again the symmetrical patterns of line 3 and line 4 have higher sum than the other patterns when the same energy is used. For instance, the patterns 13, 32, and 33 have roughly the same energy, but the sum of 33 is larger than the sum of 32, which is larger than the sum of 13. This also matches with the rating of the Fifty-five persons. We can also see that the patterns 11 and 21 have lower sum than the other patterns. Fig. 12 shows the distance between the energies of the first two levels. The symmetrical patterns of line 3 and line 4 have lower distance than the other patterns when the same energy is used. For instance, the patterns 13, 32, and 33 have roughly the same energy, but the distance of 33 is lower than the distance of 32, which is lower than the distance of 13. The patterns 41, 42, and 43 also have roughly the same energy, but the distance of 43 is lower than the distance of 42; however, 42 has higher distance than 41. We can also see that the patterns 11 and 21 have higher distance than the other patterns. These results show a close match with the rating given by the Fifty-five persons. 10

Fig. 9. Patterns from the set proposed in [46, 47], ordered from not beautiful (left) to beautiful (right) line by line. Fig. 10. The energy and the entropy of the first level of the images in Fig.9. 11

Fig. 11. The sum of the entropies of the first two levels of the images in Fig.9. Fig. 12. The distance between the energies of the first two levels of the images in Fig. 9. To give a more intuitive analysis for the proposed approach, we will take two extreme cases, the first one is an image with only one colour, and the second one is an image with equal probabilities for all colours. The first case will produce a distribution of one pulse at one energy level, while the second case will produce a flat distribution. In the case of music the first case will give a piece with only one note repeated many times, and the second case will produce a piece with all possible notes, in both cases no aesthetically appealing patterns will be produced, where the first pattern will be too regular and the second one will be too random. The aesthetically appealing patterns represent a balance between these two extreme cases, and the closer we get to the Maxwell-Boltzmann distribution, the higher the aesthetic score of the pattern. Now if we take one aesthetically appealing pattern and rearrange the pixels randomly, we will get a random pattern that has the same distribution, however the gradient of this random pattern will produce a distribution closer to the flat distribution than the gradient of the original pattern. Similarly, if we arrange the aesthetically appealing pattern such that the pixels with the same values are close to each other, the gradient of the resulted pattern will produce a distribution closer to a pulse than the gradient of the original 12

pattern. And again the distribution of the gradient of aesthetically appealing patterns represents a balance between these two extreme cases, and the closer we get to the Maxwell-Boltzmann distribution, the higher the aesthetic score of the pattern. One limitation of the proposed approach is that few aesthetically appealing patterns show lower M value and higher Md value than the less aesthetically appealing patterns as can be seen in Fig.8. Future work will improve the proposed model to increase the classification accuracy. Conclusion A novel approach to classify aesthetically appealing images was presented in this paper. The proposed approach showed that aesthetically appealing images deliver higher amount of information over multiple levels in comparison with less aesthetically appealing images when the same amount of energy is used. The results have shown that the proposed approach was able to classify aesthetically appealing patterns. Future work will try to apply this approach on other types of images. References [1] A. Chatterjee, Neuroaesthetics: a coming of age story, Journal of Cognitive Neuroscience, vol. 23, no. 1, pp. 53 62, 2011. [2] H. Leder, B. Belke, A. Oeberst, and D. Augustin, A model of aesthetic appreciation and aesthetic judgments, British Journal of Psychology, vol. 95, no. 4, pp. 489-508, 2004. [3] K. Hammermeister, The German aesthetic tradition, Cambridge University Press, 2002. [4] T. Gracyk, Hume s aesthetics, Stanford encyclopedia of Philosophy, winter 2011. [5] D. Burnham, Kant s aesthetics Internet encyclopedia of philosophy, 2001. [6] J. Shelley, The concept of the aesthetic, Stanford encyclopedia of Philosophy, spring 2012. [7] E. A.Vessel, and N. Rubin, Beauty and the beholder: highly individual taste for abstract but not real-world images, Journal of Vision, vol. 10, no. 2, 2010. [8] J. McCormack, Facing the future: Evolutionary possibilities for human- machine creativity, Springer, pp. 417 451, 2008. [9] W.H. Latham, S. Todd, Computer sculpture, IBM Systems Journal, vol. 28, no. 4, pp. 682 688, 1989. [10] R. Datta, D. Joshi, J. Li, and JZ. Wang, Studying aesthetics in photographic images using a computational approach, ECCV, 2006. [11] Y. Ke, X. Tang, and F. Jing, The design of high-level features for photo quality assessment, CVPR, pp. 288-301, 2006. [12] TO. Aydın, A. Smolic, and M. Gross, Automated aesthetic analysis of photographic images, IEEE transactions on visualization and computer graphics, vol. 21, no. 1, pp. 31-42, 2015. [13] S. Bhattacahrya, R. Sukthankar, and M. Shah, A framework for photo-quality assessment and enhancement based on visual aesthetics, in Proc. of the international conference on Multimedia, pp. 271 280, 2010. [14] Y.-J. Liu, X. Luo, Y.-M. Xuan, W.-F. Chen, and X.-L. Fu, Image retargeting quality assessment, Computer Graphics Forum (Proc. of Eurographics), vol. 30, no. 2, pp. 583 592, 2011. [15] L. Liu, Y. Jin, and Q. Wu, Realtime aesthetic image retargeting, pp. 1 8, 2010. [16] L. Liu, R. Chen, L. Wolf, and D. Cohen-Or, Optimizing photo composition, Computer Graphics Forum, vol. 29, 2010. [17] P. O Donovan, A. Agarwala, and A. Hertzmann, Color compatibility from large datasets, ACM Transactions on Graphics (Proc. of SIGGRAPH), vol. 30, no. 4, 2011. [18] D. Cohen-Or, O. Sorkine, R. Gal, T. Leyvand, and Y.-Q. Xu, Color harmonization, SIGGRAPH, pp. 624 630, 2006. [19] M. Nishiyama, T. Okabe, I. Sato, and Y. Sato, Aesthetic quality classification of photographs based on color harmony, in Proc. of CVPR, pp. 33 40, 2011. [20] S. Dhar, V. Ordonez, and T. L. Berg, High level describable attributes for predicting aesthetics and interestingness, in IEEE Conference on Computer Vision and Pattern Recognition (CVPR), pp. 1657 1664, 2011. [21] X. Lu, Z. Lin, H. Jin, J. Yang, and J. Z. Wang, Rapid: Rating pictorial aesthetics using deep learning, in Proc. ACM Int. Conf. Multimedia, pp. 457 466, 2014. [22] Y. Kao, C. Wang, and K. Huang, Visual aesthetic quality assessment with a regression model, in Proc. IEEE Int. Conf. Image Process., pp. 1583 1587, 2015. 13

[23] X. Lu, Z. Lin, X. Shen, R. Mech, and J. Z. Wang, Deep multi-patch aggregation network for image style, aesthetics, and quality estimation, in Proc. IEEE Int. Conf. Comput. Vis., pp. 990 998, 2015. [24] L. Mai, H. Jin, and F. Liu, Composition-preserving deep photo aesthet- ics assessment, in Proc. IEEE Conf. Comput. Vis. Pattern Recognit., pp. 497 506, 2016. [25] G. Birkhoff, Aesthetic Measure, Harvard University Press, 1933. [26] H.J. Eysenck, An experimental study of aesthetic preference for polygonal figures, The Journal of General Psychology, vol. 79, no. 1, pp. 3 17, 1968. [27] H.J. Eysenck, The empirical determination of an aesthetic formula, Psychological Review, vol. 48, no. 1, 1941. [28] H.J. Eysenck, The experimental study of the good gestalt a new approach, Psychological Review, vol. 49, no. 4, 1942. [29] MA. Javid, T. Blackwell, R. Zimmer, and MM. Al-Rifaie, Correlation between Human Aesthetic Judgement and Spatial Complexity Measure, International Conference on Evolutionary and Biologically Inspired Music and Art, pp. 79-91, Mar, 2016. [30] H.W. Franke, A cybernetic approach to aesthetics, Leonardo, vol. 10, no. 3, pp. 203 206, 1977. [31] B. Manaris, J. Romero, P. Machado, D. Krehbiel, T. Hirzel, W. Pharr, and RB. Davis, Zipf's law, music classification, and aesthetics, Computer Music Journal, vol. 29, no. 1, pp. 55-69, 2005. [32] R. Arnheim, Art and visual perception: A psychology of the creative eye, Univ of California Press, 1954. [33] R. Arnheim, Towards a psychology of art/entropy and art an essay on disorder and order, The Regents of the University of California, 1966. [34] R. Arnheim, Visual thinking, Univ of California Press, 1969. [35] N. Murray, L. Marchesotti, and F. Perronnin, AVA: A large-scale database for aesthetic visual analysis, IEEE conference on Computer Vision and Pattern Recognition (CVPR), pp. 2408-2415, Jun 2012. [36] Y. Ke, X. Tang, and F. Jing, The design of high-level features for photo quality assessment, In CVPR, 2006. [37] R. Datta, D. Joshi, J. Li, and J. Z. Wang, Studying aesthetics in photographic images using a computational approach, In ECCV, pp. 7 13, 2006. [38] T. D. Muller, P. Clough, and B. Caput, Experimental evaluation in visual information retrieval, the information retrieval series, Springer, 2010. [39] W. Luo, X. Wang, and X. Tang, Content-based photo quality assessment, In ICCV, 2011. [40] https://drive.google.com/open?id=1qcqaoq9yshu-tfa-eamd_fdxoy-9kfqf [42] A. M. Khalili, On the mathematics of beauty: beautiful images, arxiv preprint arxiv:1705.08244, 2017. [43] L. Boltzmann, "Über die Beziehung zwischen dem zweiten Hauptsatz der mechanischen Wärmetheorie und der Wahrscheinlichkeitsrechnung respektive den Sätzen über das Wärmegleichgewicht." Sitzungsberichte der Kaiserlichen Akademie der Wissenschaften in Wien, Mathematisch-Naturwissenschaftliche Classe. Abt. II, 76, 1877, pp. 373 435. Reprinted in Wissenschaftliche Abhandlungen, vol. II, p. 164 223, Leipzig: Barth, 1909. [44] J.C. Maxwell, "Illustrations of the dynamical theory of gases. Part I. On the motions and collisions of perfectly elastic spheres," The London, Edinburgh and Dublin Philosophical Magazine and Journal of Science, 4th Series, vol.19, pp.19-32, 1860. [45] J.C. Maxwell, "Illustrations of the dynamical theory of gases. Part II. On the process of diffusion of two or more kinds of moving particles among one another," The London, Edinburgh and Dublin Philosophical Magazine and Journal of Science, 4th Series, vol.20, pp.21-37, 1860. [46] T. Jacobsen, Beauty and the brain: culture, history and individual differences in aesthetic appreciation, Journal of anatomy, vol. 216, no. 2, pp. 184 191, 2010. [47] T. Jacobsen, L. Hofel, Aesthetic judgments of novel graphic patterns: analyses of individual judgments, Perceptual and motor skills, vol. 95, no. 3, pp. 755 766, 2002. [48] A. Gartus, H. Leder, The small step towards asymmetry: Aesthetic judgment of broken symmetries, i-perception, vol. 4, pp. 361 364, 2013. [49] L. Hofel, T. Jacobsen, Electrophysiological indices of processing symmetry and aesthetics: A result of judgment categorization or judgment report?, Journal of Psychophysiology, vol. 21, no. 1, pp. 9-21, 2007. [50] P. P. L. Tinio, H. Leder, Just how stable are aesthetic features? Symmetry, complexity and the jaws of massive familiarization, Acta Psychologica, vol. 130, pp. 241 250, 2009. [51] P. P. L. Tinio, A. Gartus, H. Leder, Birds of a feather...generalization of facial structures following massive familiarization, Acta Psychologica, vol. 144, no. 3, pp. 463 471, 2013. 14